The current application addresses two NIDA/NIMH priorities: 1) the applicant is a new investigator, and 2) the application is in response to the RFA "The Impact of Child Psychopathology and Childhood Interventions on Subsequent Drug Abuse." This new investigator proposes to conduct secondary analyses on a data set of youths approved for emergency psychiatric hospitalization and followed for 2.5 years. Data for the proposed secondary analyses were collected on an NIMH-funded randomized clinical trial of Multi-systemic Therapy (MST) as an alternative to psychiatric hospitalization (MH51852, Henggeler, PI). The data from this trial provide an excellent opportunity to address the key questions posed by the RFA (i.e., to examine the relationship between childhood psychopathology and behavior problems and later substance use disorders, and to examine the impact of childhood mental health interventions on modifying risk for subsequent substance use disorders) for several reasons: (1) Data were collected on 156 youths with significant psychopathology. (2) Data were collected at six points in time during the 2.5-year window, when many participants initiated substance use and escalated substance use. (3) This study employed an extensive substance use/abuse measurement battery, including substance use disorder diagnosis, self-reported drug use, urine drug screens, substance abuse service utilization, and substance-related arrests. (4) Retention was close to 100% throughout the study. (5) A wealth of other outcome measures was collected, including measures of youth psychopathology, caregiver substance use and psychiatric symptoms, family functioning, school attendance, mental health service utilization and official arrest data. (6) MST favorable mental health and substance outcomes in this trial support the logic of examining the degree to which psychopathology mediates or moderates MST effects on substance use/abuse. Aim 1. To identify factors linked to the initiation and progression of substance use and the development of abuse among youths presenting with serious mental health problems. Aim 2. To examine the direction of effects between psychopathology and substance use/abuse. Aim 3. To examine the effects of MST on substance use/abuse, and to explore the degree to which psychopathology mediates and moderates any effects.